Water ages in the critical zone of long-term experimental sites in northern latitudes

Abstract

Abstract. As northern environments undergo intense respond due to a warming climate and altered land use practices, there is an urgent need for improved understanding of the impact of atmospheric forcing and vegetation on water storage and flux dynamics in the critical zone. We therefore assess the age dynamics of water stored in the upper 50 15 cm of soil, and in evaporation, transpiration or recharge fluxes at four soil-vegetation units of podzolic soils in the northern latitudes with either heather or tree vegetation (Bruntland Burn in Scotland, Dorset in Canada, and Krycklan in Sweden). We derived the age dynamics with the physically based SWIS (Soil Water Isotope Simulator) model, which has been successfully used to simulate the hydrometric and isotopic dynamics in the upper 50 cm of soils at the study sites. The modelled subsurface was divided into interacting fast and slow flow domains. We tracked each day's 20 infiltrated water through the critical zone and derived forward median travel times (which show how long the water takes to leave the soil via evaporation, transpiration or recharge), and median water ages (to estimate the median age of water in soil storage or the evaporation, transpiration and recharge fluxes). Resulting median travel times were time-variant, mainly governed by major recharge events during snow melt in Dorset and Krycklan or during the wetter winter conditions in Bruntland Burn. Transpiration travel times were driven by the vegetation growth period with 25 longest travel times (200 days) for waters infiltrated in early dormancy and shortest travel times during the vegetation period. However, long tails of the travel time distributions in evaporation and transpiration revealed that these fluxes comprised waters older than 100 days. At each study site, water ages of soil storage, evaporation, transpiration and recharge were all inversely related to the storage volume of the critical zone: water ages generally decreased exponentially with increasing soil water storage. During wet periods, young soil waters were more likely to be 30 evapotranspired and recharged than during drier periods. While the water in the slow flow domain showed long-term seasonal dynamics and generally old water ages, the water ages of the fast flow domain were generally younger and much flashier. Our results provide new insights into the mixing and transport processes of soil water in the upper layer of the critical zone, which is relevant for hydrological modelling at the plot to catchment scales as the common assumption of a well-mixed system in the subsurface neither holds for the evaporation, transpiration nor recharge. 35Hydrol. Earth Syst. Sci. Discuss., https://doi.org/10.5194/hess-2018-144 Manuscript under review for journal Hydrol. Earth Syst. Sci. Discussion started: 27 March 2018 c Author(s) 2018. CC BY 4.0 License. 2 IntroductionWater ages are useful metrics to assess hydrological processes as they reveal interactions between storage and fluxes of water in a hydrological system (Hrachowitz et al.,...

The Demographics of Water: A Review of Water Ages in the Critical Zone

“…As the upper soil layers usually contain relatively young waters (see section ), the soil evaporation flux should also contain young water. Soil physical simulations indicate that the mean ages of evaporative fluxes range between 1 and about 50 days at sites in northern latitudes (Sprenger, Tetzlaff, Buttle, Buttle, Laudon, & Soulsby, ). Generally, the partitioning between evaporation and transpiration flux is challenging, but increasingly applied in situ stable isotope measurements were shown to allow distinguishing between both fluxes (Wang et al, ).…”

Waters flowing out of systems are younger than the waters stored in those same systems

“…As a consequence, soil water is a mixture of previous events with different isotope compositions and different ages. Analytical tools to infer ages of water leaving soils (e.g., Kim et al, ; Sprenger, Tetzlaff, Buttle, Laudon, & Soulsby, ; Benettin, Queloz, Bensimon, McDonnell, & Rinaldo, ) parallel to those used in catchments (e.g., Harman, ) reflect how rainfall mixes with stored waters. Of course, age contrasts only manifest in isotopic differences if precipitation isotope ratios vary over time so age can be inferred.…”

“…Therefore, we need to move beyond reporting isotopic differences between stores, and instead, we should use these isotopic contrasts between plants and streams to infer the ages of waters stored in soils and of those flowing through them. Such analysis can be completed using detailed models with intensive data requirements (e.g., Harman, ; Benettin et al, ; Knighton et al, ; Evaristo et al, ; Sprenger, Tetzlaff, Buttle, Laudon, Soulsby, et al, ) or using simple point measurements and back of the envelope calculations (e.g., Ehleringer et al, ; Allen, Kirchner, et al, ). Examining these age contrasts ultimately supports better system understanding and more accurate predictions.…”

Climate-phenology-hydrology interactions in northern high latitudes: Assessing the value of remote sensing data in catchment ecohydrological studies

“…In terms of vegetation impacts on catchment hydrology, previous studies have shown that a prolonged growing season can enhance plant growth and thus increase transpiration (Berninger, 1997;Hwang et al, 2014). In these low-energy northern catchments, soil evaporation is limited, and transpiration contributes most of the total evapotranspiration (Sprenger et al, 2018;Wang et al, 2018). Increases in transpiration can result in a decline in streamflow (Deutscher et al, 2016;Kim et al, 2018).…”